Elsevier

Applied Surface Science

Volume 483, 31 July 2019, Pages 219-230
Applied Surface Science

Full length article
Fabrication of Pd-TiO2 nanotube photoactive junctions via Atomic Layer Deposition for persistent pesticide pollutants degradation

https://doi.org/10.1016/j.apsusc.2019.03.285Get rights and content

Highlights

  • Fabrication of Pd-TiO2 catalytic junctions by Atomic Layer Deposition (ALD).

  • Accurate optimisation of junction morphology and catalytic interface.

  • Evaluation of catalyst morphology by Small Angle X-ray Scattering.

  • Impact of surface chemistry on visible light sensitization and Surface Plasmon Resonance.

  • Optimisation of nano-structured catalyst for the treatment of Persistent Organic Pollutants.

Abstract

The design of nano-structured heterogeneous catalytic junctions with high interface to volume ratio and discrete surface distribution is critical to promote the photoelectron activity in the catalytic degradation of organic pollutants. In this work, photocatalytic palladium‑titanium dioxide nano-junctions were fabricated via Atomic Layer Deposition (ALD) of palladium nanoparticles over the surface of titanium dioxide nanotubes. The Pd catalytic interface and resulting active site density was tailored by varying the nanoparticle growth and coalescence via ALD, leading to Pd-TiO2 junctions with distinctive morphological aspects and interface properties. The visible light response of the Pd-TiO2 junctions was attributed to the Surface Plasmon Resonance effect and correlated to the variation of the catalyst morphology tuned by ALD. Uniform, discrete distribution of Pd nanoparticles with diameter lower than 5 nm led to high catalytic interface to deposited volume ratio. The nano-engineered Pd-TiO2 junctions showed enhanced photocatalytic activity towards the degradation of methylene blue selected as a model contaminant and 2,4 D, with a kinetic constant 4.5 higher than as-annealed anatase TiO2 nanotubes. The design of well-defined catalytic junctions obtainable by a scalable, accurate deposition technique such as ALD represents a promising route to develop cutting-edge photoactive devices with high performance and minimum noble-metal loading.

Introduction

TiO2 nanomaterials have been deeply investigated and applied in electro and photocatalytic processes for their unique semi-conductor electronic properties [[1], [2], [3]]. Nonetheless the efficient degradation of persistent organic pollutants is still challenging, and strongly depends on the surface morphological properties such as specific catalytic surface area as well as distribution and density of active sites [4,5]. The TiO2 photo-activation is regulated by its band-gap, however the main TiO2 crystalline phases used in industrial applications, rutile and anatase, present high band-gap energies of 3 and 3.2 eV, respectively [6]. These wide band-gaps require a specific UV light source to activate the electron transfer, meaning that solar light, presenting only 5% of UV radiation, cannot be exploited as a source of activation for crystalline TiO2 [[7], [8], [9]]. Upon band-gap activation, the ability to provide a high degree of control over the separation charge carriers generated on the catalyst active sites is critical to sustain and enhance the redox reactions at the catalyst/water interface [10,11].

Different strategies have been carried out to improve the catalytic efficiency of pure TiO2. Typically, TiO2 nanotubes represent an ideal candidate as a catalytic substrate given their high specific surface area [12] and fast electron transfer rate with minimal charge recombination effect provided by the well-structured vertical alignment [13]. Nevertheless, applications in catalysis are hindered by the wide bang-gap [14,15]. Pure material doping, whereby a foreign metal or non-metal atom is introduced into the matrix, is carried out on the TiO2 nanostructures to shift the absorption towards the visible light spectra [15,16]. The introduction of the doping agent into the matrix requires however post-synthesis treatments such as plasma doping, ion implantation or, thermal annealing. These treatments may damage the TiO2 nano-architecture, resulting in loss of morphological structure such as shrinkage, sintering of adjacent nanotubes or loss in crystallinity [17,18]. Therefore requiring narrow control and limitation of the induced lattice damage [19,20].

The surface decoration of the TiO2 substrate has been considered as an alternative pathway to the doping route to improve the optical and electronic properties of the material without affecting the TiO2 lattice microstructure [21,22].

Photoactive junctions can be generated by the heterogeneous interaction with 2-D materials, such as graphene and its derivatives [23,24], or with noble metals. The design of photo-responsive junctions between semiconductors and noble metals requires however the deposition of nanoparticles (NPs) with high specific surface area and controlled morphology rather than thin coatings [[25], [26], [27]]. Specifically, noble metal NPs can enhance the optical and electronic properties upon light irradiation by exploiting the Surface Plasmon Resonance (SPR) effect [28], whereby electrons are excited and transferred at the noble metal/semi-conductor interface while the formation of a Schottky barrier prevents the charge recombination [[29], [30], [31]]. Noble metals such as Pt, Pd, Au and Ag have been investigated as particularly efficient candidates for this purpose and consequently applied in applications such as photovoltaics, H2 generation and photocatalysis [[32], [33], [34]]. Pd has gained growing attention, since it presents higher chemical stability to poisoning and corrosion than Ag [35,36] and, as Pt, possesses better thermal resistance than Au with lower risk of sintering [37,38].

Advanced nanofabrication routes are required to control the seeding of the noble nanoparticle across the surface of metal oxides, hence the interface affected by the charge transport [39]. The uniform distribution and high density of active sites, consisting of noble metal/metal oxide junctions, across the surface is critical to provide high specific surface area of catalytic sites [40,41]. The catalytic performance is directly correlated to the chemical and physical properties of the photoactive junction, therefore the deposition process is crucial to optimise size, shape and composition of the deposited nanoparticles [42,43].

Atomic Layer Deposition (ALD), a vacuum based deposition technique, has emerged as an ideal candidate to uniformly deposit NPs or thin films with controllable dimensions at the nanometre scale on challenging porous metal supports [[44], [45], [46]]. The fabrication of nano-structured catalysts by ALD offers the unique possibility of providing both homogeneous discrete dispersion and controlled particle size, hence tailoring the surface density of the catalytic active sites [47,48]. Well-organised nanoparticles can be deposited on nano-structured templates, leading to unique catalytic substrates with specific morphological aspect and interfaces [49,50]. ALD represent a scalable, rapid technology enabling the design of nanocatalysts with accurate control of distribution, size, composition and density with reduced waste and environmental impact [51,52]. This strategy is particularly attractive when considering noble metals such as Pd, since a low concentration of this expensive metal is desired to make the process cost-effective [45]. Although ALD has been used recently to prepare nanoporous noble metal-TiO2 materials for electrocatalytic applications [22,53], only a few studies are available on the potential of Pd-TiO2 nanotubes as photoactive substrates towards the degradation of POPs [54]. To this end, the impact of the morphology of Pd-TiO2 junctions and nano-interfaces on the photocatalytic activity is not yet understood and largely unexplored.

In the present work, Pd-TiO2 nanotube catalytic junctions with distinctive morphology and composition were fabricated via ALD. The physical and chemical properties of the supported Pd NPs, and therefore the catalytic interface of Pd-TiO2 junctions, were controlled by the ALD process and correlated to the catalytic performance. The impact of the SPR combined with the morphology resulting from the ALD deposition was discussed to determine the reaction mechanism occurring across the Pd-TiO2 junction. The photoenhanced degradation of 2,4 D, used as a benchmark POP, demonstrates the potential of ALD as a rising technology to manufacture nano-textured catalysts with well-defined chemical and morphological properties. The application of advanced nano-engineered materials based on Pd-TiO2 junctions can be extended from purely electro-oxidation to sun-light driven photocatalytic degradation of POPs in wastewater.

Section snippets

Materials and chemicals

Ethylene glycol (>99.8%, CAS: 107-21-1), ammonium fluoride (>99.99%, CAS: 12125-01-8), ethanol (>99.5%, CAS: 64-17-5), acetone (>99.5%, CAS: 67-64-1), palladium hexafluoroacetylacetonate (Pd(hfac)2) (CAS: 64916-48-9), formalin solution (10%, CAS: 50-00-0) were purchased from Sigma-Aldrich and used as received. Milli-Q water was used without further purification. Ti foil (thickness 0.1 mm) were purchased from GoodFellow.

Electrochemical anodization

The anodization experiments were carried out in a custom built two-electrode

Results and discussion

Different Pd-TiO2 junctions with specific morphology and interfaces were fabricated with the ALD deposition of Pd. The formation, growth and coalescence of the Pd NPs over the surface top layer of TiO2 nanotubes at different cycles were revealed from the observation of Fig. 1(A), (C) and (E). Small number of NPs exhibiting a mean diameter (D) of 5.1 nm were dispersed over the surface after 50 deposition cycles (Fig. 1(B)). The nanoparticles became larger between 50 (Fig. 1(B)) and 100 cycles (

Conclusions

The catalytic performance of Pd NPs-TiO2 nanomaterials has been investigated and discussed. The results observed showed a significant impact of the morphological, microstructural and surface chemistry of the different noble metal/semi-conductor interfaces obtained by the ALD process. The fastest degradation rate under UV–visible light and pure visible light was obtained with the Pd-TiO2 photocatalysts containing 2.7 at.% of Pd. 70% degradation of the 2,4D pesticide and 90% degradation of

Acknowledgment

This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). Mr. Andrea Merenda would like to acknowledge the Australian Research Council (ARC) for funding the Linkage LP140100374 project and Dr. Ludovic Dumée also acknowledges the ARC for his DECRA DE180100130 fellowship. Mr. Andrea Merenda and Dr. Ludovic Dumée would also like to acknowledge ARC Research Hub for Energy-efficient Separation

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